1. Field of the Invention
The present invention relates generally to nanofiber filtration and, more particularly, to a three-layer filter membrane having a top coating layer with cellulose nanofibers.
2. Description of the Related Art
The present application is related to Publication No. US 2009/0078640 A1 of U.S. patent application Ser. No. 12/126,732, filed May 23, 2008, and provisional application Nos. 60/931,765 and 60/947,045 filed on May 26, 2007 and Jun. 29, 2007, respectively, the contents of each of which is incorporated herein by reference.
A unique class of nanofibrous membranes with fiber diameters of approximately 100 nanometers (nm) and lengths on the order of thousands of meters have been created by precision multi-jet electrospinning technology. This technology takes advantage of a non-woven nanofibrous structure with uniform distributions of fiber diameter and membrane pore size that can significantly improve the flux of water transport at low operating pressures without loss of selectivity. The diameter of a fiber prepared from an electrospinning technique ranges from 1 micron to 50 nm. This implies that a mean pore size of the nanofiber membrane will range from 3 microns to 150 nm, enabling its use as a microfiltration membrane. However, it is difficult for the electrospinning process to prepare a nanofiber having a diameter that is less than 50 nm.
Methods have also been developed for the fabrication of cellulose nanofibrous scaffolds from cellulosic biomass. These nanofibers have diameters of approximately 5 nm and lengths of a few micrometers (μm). The use of cellulose nanofibers for water filtration is especially advantageous because the surface of cellulose nanofibers can be functionalized to guide the flow of water inside water channels, or to selectively alter the adsorptive or repulsive properties for particulate separation. The cellulose nanofibers are mostly crystalline in nature and, unlike amorphous cellulose, they have shown to be relatively bio-inactive.
A first type of nanofibrous membrane created through electrospinning technology utilizes PolyVinyl Alcohol (PVA), preferably on a non-woven PolyEthylene Terephthalate (PET) substrate. Different concentrations (such as 6, 8, 10, 12 wt %) of PVA solutions have different effects on fiber diameter in the membrane. Due to the fact that electrospun PVA nanofibers can be dissolved in water, the electrospun PVA membrane is chemically cross-linked before use with one of many aldehydes, such as GlutarAldehyde (GA) and glyoxal. A reaction forms acetal bridges between the hydroxyl groups in PVA and the aldehyde molecules.
The maximum pore size of the electrospun membrane may be determined by a bubble-point method, which is based on a pressure measurement that is necessary to blow air though a liquid-filled membrane. Water is preferably used as the wetting reagent. The relationship between maximum pore size (d) and the corresponding pressure is given by Young-Laplace Equation (1):
                    d        =                              4            ⁢            γcosθ                                Δ            ⁢                                                  ⁢            p                                              (        1        )            
A schematic diagram of the bubble point test set-up is shown in FIG. 1. An immersed electrospun PVA membrane is placed in a membrane cell 102 having a diameter of 1.2 inches. A syringe 104 is connected to one end of the cell to provide the gas pressure, and a pressure gauge 106 is connected to the other end to monitor the pressure. A plastic tube is connected to the pressure gauge and inserted within a water filled beaker 108 to observe the air bubble. When the membrane is completely wetted by liquid, cos θ=1, and γ is the surface tension of the membrane. The minimum pressure that blows the first air bubble is recorded, and related to the maximum pore size of the membrane.
The pure water flux of the electrospun membranes is characterized using a dead-end filtration set-up, as shown in FIG. 2. A water tank 202 is placed at the water level located at 1.6 meters higher than a membrane cell 204. Therefore, it provides a differential pressure of 2.28 psi higher than gravity. The pressure was kept within 1% deviation by adding water periodically to the tank for all the measurements. Set-up of a rejection test is achieved by replacing the pure water in the flux test by a polycarboxylate feed solution.
As illustrated in FIG. 3, the average fiber diameter is reduced to 140 nm at 28 kV and 100 nm at 32 kV in electrospinning. The decrease in the average fiber diameter could be attributed to the greater elongation force provided by the increase in the electric field strength. FIG. 4 illustrates that viscosity values for PVA solutions were found to increase as the concentration increased. Specifically, there was a sharp increase in the viscosity from 50 cp to 669 cp when the concentration was increased from 10% to 12%.
FIGS. 5(a)-(d) show a series of Scanning Electron Microscope (SEM) images in order to illustrate the effect of concentration of PVA solutions on the morphological appearance of the electrospun membranes. At a low concentration of 6% or low viscosity of 16 cp, only a few nanofibers were produced, and a large number of microdroplets were formed creating a porous film-like structure. As the concentration was increased to 8% and 10%, beads gradually became less and were eliminated at 10%, whereby a uniform fiber-structure with the fiber diameter of 100 nm was formed. With a further increase in concentration to 12%, beads were formed again in the structure, and the fiber diameter increased to 150 nm.
The porosity of electrospun PVA membranes fabricated using different PVA concentrations is shown FIG. 6. At a concentration of 6%, the porosity of the membrane is quite low, 57%. Other membranes all exhibited porosity higher than 75%, and the largest reached 83% at 10% concentration.
Using a 10% PVA solution, 32 kV for the electrospinning, and membranes electrospun into sheets of 20 cm (width)×30 cm (length) at different thicknesses ranging from 3 μm to 35 μm, properties of the membranes are listed in Table 1. The pure water flux of Millex-GS is in the range of 1300-1400 (L/m2 h), with average pore size determined by the image analysis of multiple SEM images, sampled at different membrane locations.
TABLE 1Thickness (μm)35815182435Porosity85%84%85%85%85%85%83%Maximum 21.014.08.43.22.81.40.4pore size (μm)Average 7.04.72.81.10.90.50.1pore size* (μm)Flux 1640014900116008500820075005900(L/m2h)
FIG. 7 illustrates that with a thickness of 8 μm, a membrane exhibits a rejection of 89% to 0.2 μm microparticles. With the membrane being thicker, the rejection increased to higher than 95% and reached 98% as a highest value. Therefore, the rejection of electrospun PVA membranes is also affected by the thickness of the membrane.
A second type of nanofibrous membrane created through electrospinning technology utilizes PolyAcryloNitrile (PAN) solutions. Different wt % PAN solutions are prepared by dissolving PAN powder in DiMethylFormamide (DMF) and stirring the solution at 60° C. for 2 days until homogeneous. PAN/DMF is preferably electrospun directly onto a PET substrate in an electrospinning machine.
TABLE 2SolutionApparatusConcentrationDistanceVoltageE-field strengthSample(wt %)(cm)(kV)(kV/cm)A-146183.0 ± 0.1A-2411252.3 ± 0.1A-3419301.6 ± 0.1A-4424291.2 ± 0.1A-566142.3 ± 0.1A-6611171.6 ± 0.1A-7619261.4 ± 0.1A-8624281.2 ± 0.1A-986132.2 ± 0.1A-10811161.5 ± 0.1A-11819231.2 ± 0.1A-12106122.0 ± 0.1A-131011141.3 ± 0.1A-141019211.1 ± 0.1B-146183.0 ± 0.1B-246254.2 ± 0.1B-346305.0 ± 0.1B-466152.5 ± 0.1B-566203.3 ± 0.1B-666254.2 ± 0.1
Dead-end filtration cells are used for bubble point testing on the membranes, in order to determine the maximum pore size. The Young-Laplace Equation shown in Equation (1), after substitution, is used to determine maximum pore size. Bulk porosity of the electrospun membrane is calculated by Equation (2), where ρ is density of electrospun PAN and ρ0 is density of PAN powder:Bulk porosity=(1−ρ/ρ0)×100%  (2)
FIGS. 8(a) and 8(b) illustrate that both dynamic viscosity and conductivity increase with increased solution concentration. According to FIGS. 9, 10 and 11, nanofibrous membranes having higher concentrations have a fairly constant larger fiber diameter, without beads or melting parts. Higher conductivity increases the stretching forces of the electric field on the polymer, which typically results in a decreased fiber diameter. However, the increasing intermolecular forces of the polymer solution, represented by increasing viscosity, counteract the stretching of the polymer. Additionally, at higher concentrations, the polymer chains entangle to a greater degree and viscoelastic properties of the solution favor thicker fiber formation, and not beads. 2 wt % is likely to be close to the critical overlap concentration (1.8%) for PAN, which prevents the stable formation of fibers.
With increasing voltage, the variability of the fiber diameters increases, as shown in FIGS. 12(a) and 12(b). Because increasing the voltage has the same effect as increasing solution conductivity, it is believed that the decreasing uniformity is due to electron repulsion.
FIG. 13 illustrates the effects of average fiber diameter, controlled by PAN concentration, on both maximum pore size and pure water flux. For a given membrane thickness and amount of polymer, the fiber length decreases with increasing fiber diameter per unit area, resulting in a lower number of fiber crossings. A decreased number of fiber crossings reduces the amount of times that pores are defined per unit area, thereby increasing the pore size. This is also consistent with the fact that smaller fiber diameter will provide a smaller pore size. Flux and bubble point data for electrospun membranes of various thicknesses, prepared from 6 wt % solution, are listed in Table 3.
TABLE 3Pure Water Pure Water Flux after AverageMaximumFluxEthanol Pre-treatmentFiberMembranePore(L/m2 h)(L/m2 h)DiameterThickness Size1st5th1st5thSample #(nm)(μm)(μm)MinuteMinuteMinuteMinuteE-1110 ± 20101.771002400100006000E-2110 ± 20200.956002000 68004100E-3110 ± 30300.744002800 57003400Millex-GSN/AN/A0.714001300 14001300
The smallest maximum pore size attained was 0.7 μm by sample E-3, which is equal to that of a Millipore Millex-GS™ microfiltration membrane. Pure water flux rates for this sample were three times higher than those of Millex-GS during the 1st minute and two times higher in the 5th minute. Sample E-3 exhibited the highest rejection out of all of the electrospun membranes produced, which was predicted by bubble point results. Compared to Millex-GS, it showed significantly higher flux (2800 to 800) at comparable rejection of 1 μm particles, as shown in FIG. 14. During filtration of 0.20 μm particles, electrospun PAN performed significantly better in both flux (2600 to 700) and rejection (90% to 25%).
Cellulose nanofibers are new nano-scale materials, which can be prepared from natural plants after chemical and mechanical treatments. Nano-scale cellulose-based fibers have many applications because of their smaller diameters and the ability for surface modifications. Advantages of cellulose nanofibers over other nano-scale materials are set forth below.
(1) The diameter of cellulose nanofiber is very small, usually only ˜5 nm, implying higher surface area (about 600 m2/g) and higher slip flow for gas (e.g., air) filtration.
(2) The surface of cellulose nanofibers is very hydrophilic since there is one primary hydroxyl group (12 mol % or more can be transferred into carboxyl groups) and two secondary hydroxyl groups, which can be utilized to change the hydrophilic nature of the surfaces and thereby to construct liquid nano-channels.
(3) Highly functionalized surface of cellulose nanofibers means that the chemical modification can be performed more easily to achieve multiple functions, such as charged or chelating properties.
(4) Biocompatibility of cellulose nanofibers is very good, which permits biomedical applications. For long term use, such as in hemodialysis, the complementary reactions have to be properly taken into account, e.g., by reducing the active groups on cellulose.
(5) Cellulose nanofiber aqueous solutions are pH sensitive and ionic strength sensitive, permitting the formation of new gel-like structures.
(6) The low concentration of cellulose nanofibers in an aqueous solution can be utilized to prepare membranes with very thin barrier layers, useful for low-pressure micro-filtration, ultra-filtration, nano-filtration, and pre-filtration in reverse as well as forward osmosis.
(7) Cellulose nanofibers can be fabricated from cellulose under environmentally benign conditions, including the production of bacterial cellulose.
(8) Cellulose nanofibers with oxidized carboxyl groups have anti-bacterial properties. In addition, the surface property can be modified to resist interaction with bacteria.
(9) Initial source materials for the preparation of cellulose nanofibers are relatively cheap and easily available from natural plants.
The conventional preparation of cellulose nanofibers includes pre-treatment (swelling with alkali aqueous solution) of cellulose fiber bundles, acid hydrolysis to remove pectin and hemicellulose, alkali treatment again to remove lignin, high impacted cryo-crushing to liberate the microfibril from the cell wall, and high impacted and high sheared defibrillation to obtain the individual nanofibers, as shown in FIG. 15.
The diameter of cellulose nanofibers prepared by the above method is about 10 to 100 nm and having a yield of about 20%. Moreover, many of the steps often used highly corrosive reagents, such as strong acids and alkali. The cryo-crushing and defibrillation processes require special devices, which can seriously affect the extension of this method for large scale operations.
One benign preparation of cellulose nanofibers is the production of Bacterial Cellulose (BC) nanofibers using acetobacter xylinum. BC fibers have a network structure with diameters in the 10 to 70 nm range and excellent physical properties.
Physical preparation of cellulose-based nanofibers can also be achieved using the electrospinning technology. A cellulose solution can be prepared using an ionic liquid, such as 1-butyl-3-methylimidazolium chloride, N-methylmorpholine-N-oxide, or a mixture of solvents. Alternatively, cellulose acetate nanofibers are hydrolyzed, as fabricated by the electro-spun method, by using an alkali aqueous solution. However, such cellulose nanofibers have higher fiber diameter in an approximate range of 300 to 1000 nm, and the process includes an additional post-treatment step using either a toxic or volatile reagent.
Membranes suitable for filtration which involve one or more of the above technologies can be found in International Publication Nos. WO 2005/0049102 and WO 2007/001405.